LT1 Metering Block

[Gregory G.]

One year ago I had my Holley 4555 carburator restored or Chevrolet #3972121.When I had it judged it was discovered that the primary metering block was a 6328 instead of the 6333.The car runs perfect,but I want to install the proper one, which I do have, it is a restored piece sent to me by the person who restored the carburator.It was sent without the power valve,air mixture screws and gaskets.Is it simple enough to remove the 6328 metering block from the carburator removed from the intake manifold and swap out the air mixture screws and the power valve, the size of which I do not know 6.5 maybe the correct application, but I also heard an 8.5 works alot better?The 6328 metering block maybe for a 396 LS6 chevelle motor.What is the difference between the 2 metering blocks?

[Jim T.]

Re: LT1 Metering Block

Greg the power valve is something I would replace with a new one if you change metering blocks on your LT-1. Holley power valves are available individually as well as metering block gaskets and fuel bowl gaskets.
Power valve differences are measured in inches of vacuum. A 6.5 valve will open when the vacuum drops below 6.5 inches and a 8.5 will open when the vacuum drops below 8.5 inches. Recommend you use what originally came with the LT-1. I use the 6.5 power valve on my 68's 327/350 HP and because of the lower vacuum at idle on my 70's crate engine ZZ engine I use even a lower vacuum opening power valve.

[Jeremy D.]

Re: LT1 Metering Block

Putting on the 6333 is simple....be sure to use a 6.5 power valve for starters, and make sure you have the factory jets installed ( I think it was #62 primary, #68 secondary, but don't quote me, that's off the top of my head.) And MAKE SURE YOU USE NEW GASKETS....HOLLEY GASKETS for both the metering block to float bowl and the metering block to carb body. I don't know the difference between the blocks, but I'd bet on having to reset your float level as well.....make sure you crack the secondary butterflies open and let the motor gain RPM's before you check it on the secondary side.

[Dick W.]

Re: LT1 Metering Block

Last, but not least, do NOT overtighten the block. Big screwdrivers and a heavy hand cause most of the Holley problems.

[Gregory G.]

Re: LT1 Metering Block

Who would be the best source for correct original style Holley parts? In previous tech discussions I read Chicago Corvette,Bob Kunz or Jerry Luck to name a few. For one Holley does not make the small slotted air mixture screws as I am use to seeing they are a large slotted pan head screw.Several local speed shops here in N.Y. carry the red color gaskets.They even looked up in the Holley parts book that the primary calls for an 8.5 power valve.Any other info would be appreciated.